M'k I know a great way to earn credits but only if you are a warrant officer or lower. Go to Firefight, Go to firefight options, Spartan Settings, then weapons, Use a Fuel Rod cannon and a rocket launcher change shield regeneration to I think 200% same with health. put difficulty on Legendary. Play until bonus round and you get pretty much 1,000 Cr. If you are a Lt. something then you only get like 160 Cr. So there's a great way to earn credits if you are a noob to Halo reach. Good Luck!